639691 has 4 divisors (see below), whose sum is σ = 688912. Its totient is φ = 590472.

The previous prime is 639689. The next prime is 639697. The reversal of 639691 is 196936.

639691 is digitally balanced in base 2, because in such base it contains all the possibile digits an equal number of times.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 639691 - 2¹ = 639689 is a prime.

It is a Duffinian number.

It is a nialpdrome in base 15.

It is not an unprimeable number, because it can be changed into a prime (639697) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 24591 + ... + 24616.

It is an arithmetic number, because the mean of its divisors is an integer number (172228).

2⁶³⁹⁶⁹¹ is an apocalyptic number.

639691 is a deficient number, since it is larger than the sum of its proper divisors (49221).

639691 is a wasteful number, since it uses less digits than its factorization.

639691 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 49220.

The product of its digits is 8748, while the sum is 34.

The square root of 639691 is about 799.8068516836. The cubic root of 639691 is about 86.1635161953.

The spelling of 639691 in words is "six hundred thirty-nine thousand, six hundred ninety-one".